February 18, 2012

Chapter 21

The atom H, the first on the periodic table of atoms is the most commonly found atom in the universe. In the human body which contains as much as 60 or 70% water, the H atom (H2O) is also present in large numbers.

The Hydrogen atom has one proton in its nucleus (the positively charged particle), and only one electron revolving around it. Like all sub-atomic particles the electrically charged proton spins around itself and therefore creates an electro-magnetic field around it.

In normal circumstances the axis of spinning of different protons are spread in a helter-skelter manner in different directions .However, any external magnetic influence changes this situation and such an application will create a new arrangement where most of the protons will align along the magnetic field of the external magnet (parallel) and few protons align in the opposite direction (antiparallel). The parallel and antiparallel forces cancel out each other with a net resultant vector along the external magnetic field which is called the longitudinal magnetization. In addition to spinning around its own axis, in an external magnetic field the axis of protons also wobbles or spins along the direction of the external magnetic field with a particular frequency called precession frequency.

Atoms with protons in odd numbers are more susceptible to this kind of longitudinal magnetization. Hydrogen has only one proton, an odd number therefore this law prevails and because water has hydrogen as its component, water is susceptible to this magnetization. Water (free) or water bearing fluids or water in the form of edema or soft tissue with a certain water content are influenced by this external magnetization.

The MRI or Magnetic Resonance Imaging machine is a tubular magnet in which a patient (or an individual) is placed. This leads to longitudinal magnetization in the body tissues of the patient.

Each proton has a certain precession frequency depending on the nature of tissue and the strength of the external magnetic field. If therefore an appropriate radiofrequency pulse, (which resonates with the precession frequency of the proton) radiofrequency pulse perpendicular to the external magnetic field is sent to the body while it is in the magnet the protons get charged and the extra energy that they get results in their movement. This movement alters the longitudinal axis and makes it transverse and the protons now have longitudinal as well as transverse magnetization. Some or many protons are now in a sense aligned in a perpendicular direction to their longitudinal magnetization.

Once the radio frequency pulse is switched off the protons move back to their longitudinal axis. Therefore there is recovery of longitudinal magnetization and decay or decrease of transverse magnetization. This process is not instantaneous but occurs at a certain rate and this rate is determined by the surrounding tissue and the strength & homogeneity of the external magnetic field.

The return to the longitudinal axis (recovery of longitudinal magnetization) is called T-1 and the image obtained in the process is called a T-1-weighted image.

In contrast the demise or decay of the transverse magnetization can also be charted and is called T-2 and the images obtained are called T-2-weighted images.

These changes are received by radio-sensors called coils and this information is processed by a computer which has a software which is equipped to chart these signals. The software is a product of observation of experiments on different tissues and their reaction to various radiofrequency signals with echoes of either the T-1 or T-2 variety or other combinations.

These radiofrequency pulses are repeated successively to scan the entire area of interest. In addition to T-1 and T-2 many other techniques and pulse sequences are also available to optimally depict different tissues depending upon their relaxation times. The system can also use contrast material (Gadolinium, chemically a rare earth with odd number of protons). The flow within vessels can also be detected with or without the use of contrast material (MR Angiography). Also different areas of the body can be focused upon to selectively view sections of a part of a certain thickness in any desired imaging plane to diagnose the extent and the nature of pathological changes.

The above oversimplification is meant to give only some basic information to a plastic surgeon. What also needs to be noted is that the radiofrequency signals are repeated successively at different intervals, at different angles, the magnets within the chamber are of various strengths and echoes that are produced are picked up on a continuous basis by sensors and are passed on for analysis.

How different tissues appear on T-1 or T-2 weighted images :

T-1

T-2

Skin

Gray

Gray

Subcutaneous fat

White

White / Gray

←

Fascia

Black

Black

Muscle

Gray

Gray

Tendon

Gray

Gray

Nerves

Gray

Gray

Cortical Bone

Black

Black

Fatty Bone marrow

White

Gray

←

Red bone marrow

Gray

White

←

Patent Blood vessel

Black*

Black*

Blocked Blood vessel

White / Gray

White / Gray

Free water, urine, ascitic / pleural fluid, CSF

Black

White

←

Oedematous tissue

Black

White

←

Bile

Variable usually black

Variable usually white

Free blood or blood clot

Variable

Variable

Blood within vessels

Black

Black

Grey matter of brain

Gray

White

←

White matter of brain

White

Black

←

The difference in images as it occurs in T1 and T2 are marked with arrows.

The different appearance of blocked and open blood vessels on both T1 & T2 images are marked with an asterisk.

Some representative situations where an MRI is of great value are reproduced below with the help of illustrations.

Vascular Anamoly medial side of thigh, the extent of which has be properly judged by a MRI image. Photographs courtesy: Samir Kumta, Mumbai

(From left to right) 1. Adenocarcinoma arising from maxillary antrum. 2 and 3. A lesion is occupying the antrum and is pushing the orbital contents laterally and has produced proptosis. 4 The same lesion seen in the earlier pictures can be seen to be encroaching on the skull base. Picture courtesy: Nitin Mokal, Mumbai

Left: A naso-fronto-ethmoidal encephalocoele. Please note mild hypertelorism and telecanthus. Right: The lesion can be seen coming from the anterior-cranial fossa through the ethmoids. Photographs courtesy: Nitin Mokal, Mumbai

Left: Mid Line Dermoid. Right: Please notice that there is only erosion of the nasal bones but no spatial gap. Pictures courtesy: Nitin Mokal, Mumbai

Left: Mid-line sinus on the nose. Middle: The cranial base is not breached. Right: The cranial base is not breached both on coronal and horizontal sections. Arrow shows the blind end of the sinus. Pictures courtesy: Nitin Mokal, Mumbai

Left: Aborted encephalocele with a midline sinus obliterated in the midline at the septum. Right: Aborted encephalocele with a midline sinus obliterated in the midline at the septum. Pictures courtesy: Nitin Mokal, Mumbai.

Malignant, invasive neurofibroma of the right infra-temporal region causing massive local destruction and crossing over across the midline. The case is inoperable. Pictures courtesy: Nitin Mokal, Mumbai.

Left: Congenital cyst in the infra temporal region extending into the mouth. Middle: The circular cyst can be seen below the zygomatic arch. Right: The MRI shows a near pedunculated extension into the buccal cavity. Pictures courtesy: Nitin Mokal, Mumbai

Middle aged lady referred by orthopaedic surgeon for pain at back of knee radiating to the leg. No significant fullness on inspection in popliteal fossa as compared to opposite side but vague tenderness was present in popliteal fossa. MRI showed a tumor within the popliteal nerve. It was explored under tourniquet and excised under magnification keeping all nerve fascicles intact. Histopathological examination showed neurolipoma. These pictures contributed by Parag Sahasrabuddhe from Pune show the value of an MRI for a soft tissue tumor which is very well seen here on both the coronal as well as the sagittal section when there was no visible swelling and nothing was palpable by hand.

These photographs sent by Mukund Thatte from Mumbai show the advantage of MRI again in a soft tissue tumor. In these two cases of glomus tumors in fingers it was very easy to locate the tumor on an MRI and therefore surgery became that much easier as compared to the past when an x-ray usually was of little help and the clinician had to depend upon his gut feeling about the tumor which did not appear very dissimilar to its surrounding fat.

This is an unusual use of the MRI. This person had fractures of both radius and ulna which had been internally fixed. In a strange incident he fell on the same hand again, the metal used to internally fix the fractures got displaced and caused chronic attrition of the extensor of the index finger not visible on ultrasound examination due to granulation tissue but is quite evident on an MRI examination. See arrow. Photograph courtesy: Dr. Hemant Kotwal, Nashik.